Whereas the inclusion of tin oxide in lead-free and cadmium-free frits has demonstrated essentially universal effectiveness in eliminating the endemic grayish discoloration witnessed in the firing of those frits, a substantial concentration of tin oxide, generally at least 1.5% by weight, was required to assure complete removal of the discoloration. Because of the inherent cost of tin oxide, other means were investigated concurrently to accomplish the same goal, but without the need for such substantial levels of tin oxide.
Hence, the principal objective of the present invention was to devise means for treating lead-free and cadmium-free frits to assure the absence of the grayish discoloration after firing, which means would not involve the incorporation of substantial amounts of tin oxide in the frit composition.
A complementary objective was to not only achieve the above principal objective, but also, at the same time, to make certain that the resultant glazes were not so altered thereby that their physical and chemical properties, e.g., their softening point, their linear coefficient of thermal expansion, and their resistance to alkaline detergents, were not deleteriously affected.